This invention relates generally to the measurement of pipe wall thickness, and particularly to a method of measuring the average wall thickness of buried pipes, for example, municipal water pipes in a non-intrusive manner.
Buried pipes, particularly municipal water pipes, deteriorate over time. In particular, buried pipes lose wall thickness as a result of corrosion and eventually fail leading to service disruption and economic loss. The pipes have to be periodically inspected and their structural capacity evaluated against design loads. Accurate information about pipe wall thickness is needed to determine the structural capacity.
Also, monitoring of wall thickness loss is needed to estimate the remaining service life of pipes, which is an important part of effective maintenance management systems of large pipe networks, for example, urban water distribution systems.
There is a need to measure the wall thickness of buried pipes in a nondestructive and non-intrusive way, that is without damaging the pipe and without taking it out of service.
Pipe wall thickness may be measured by obtaining pipe samples, known as coupons, and then measuring the thickness off-site using an ultrasonic gauge. This method provides only a discrete measurement of wall thickness at a specific point on the pipe. An alternative method for obtaining a continuous measurement of pipe wall thickness employs ultrasonic or magnetic flux leak sensors that are launched inside pipes using robots (known as pipeline pigs).
Obtaining pipe coupons is very expensive and requires excavating the pipe at several points. Also, thickness values based on the coupons may not be statistically representative of the overall condition of the pipe unless a very large number of coupons is obtained, which is often not practical.
Pipeline pigging requires taking the pipe out of service and cleaning the pipe before launching an inspection pig. It is mostly used for large diameter pipes with simple geometry, (i.e. no abrupt bends) such as oil and gas transmission pipelines. Also, data acquisition and analysis is very intensive and expensive. Pipeline inspection pigs are therefore not suitable for water distribution systems, which usually consist of small-diameter pipes forming networks that are very complex geometrically. Also, water pipes are usually filled with debris and have no pig launching stations.
Brophy et al., U.S. Pat. No. 5,970,434, discloses a method of measuring the wall thickness of a pipeline, wherein ultrasonic or magnetostrictive wave probes are used to analyze the dispersive behaviour of waves traveling in the tube wall volume. These waves are injected into the pipe at a first location and measurements are taken at a second location. Changes in cut-off frequency, according to Brophy, are related to pipe wall thickness. Brophy""s method is designed for the testing of xe2x80x9cU-shaped tubesxe2x80x9d found industrial heat exchangers and will not work in municipal water systems since the ultrasonic waves would be damped too quickly as the ultrasonic waves attenuate very rapidly and would not therefore propagate over a sufficient distance to be useful for municipal water pipes. Also, most water systems contain pipes that are mechanically joined, and the waves would not propagate through the joints.
According to the present invention there is provided a method of measuring the wall thickness of a pipe carrying a fluid medium, comprising sensing disturbances in the pipe at two spaced locations, determining the propagation velocity of coupled propagation mode signals at low frequencies resulting from said disturbances, and calculating the average pipe wall thickness between said two locations from said propagation velocity and known constants for said pipe and fluid medium.
The coupled propagation velocity is typically in the range of 1000-1,300 m/s for metal pipes, and 300-500 m/s for plastic pipes. The invention is however applicable to other types of pipe material, such as concrete.
The advantage of using the coupled propagation mode, wherein propagation occurs both in the fluid and pipe wall, is that it is non-dispersive at low frequencies. By low frequencies, it is understood that the wavelength of the disturbances is large compared to the diameter of the pipe, typically five to ten times the pipe diameter. In coupled propagation mode, a signal propagating in the fluid medium continually excites the pipe wall, and vice versa. The two vibrational phenomena interact to propagate the wave along the pipe even past joints and other obstructions.
The locations may, for example, be spaced 100 meters apart. The difference between the times of arrival of a signal from a disturbance originating outside the two sensors permits the velocity of propagation to be calculated. In the coupled propagation mode, this velocity can be conveniently used to determine pipe wall thickness. At higher frequencies, dispersive effects and other factors make such a calculation much more complicated.
In accordance with the invention, the pipe wall thickness is determined from the average propagation velocity of low-frequency dynamic disturbances in the pipe, e.g. pressure fluctuation.
The propagation velocity can be obtained on-site non-destructively and non-intrusively based on the time shift (or lag) between vibration signals measured simultaneously at two separate points of the pipe.
Ambient noise in the pipe can be utilized as the vibration source. Alternatively, noise simulated by releasing pressurized fluid inside the pipe at an in-bracket or out-of-bracket point can be used, e.g., a fire hydrant in the case of wafer distribution pipes. Vibrations may be measured using a pair of accelerometers (or hydrophones) placed at two points that are a known distance apart and in direct contact with the pipe, for example, at control valves or fire hydrants.
The time shift between the measured signals is determined using the well-known time-of-flight method or the cross-correlation function calculated in the either of the time or frequency domains. The average propagation velocity in the pipe can then be calculated by dividing the sensor-to-sensor spacing by the time shift.
The propagation velocity depends on the type and size of pipe, i.e., its material, diameter and wall thickness. It can be calculated theoretically by the following equation:   c  =            c      o        ⁢                  1                  [                      1            +                                          a                ⁡                                  (                                      D                    /                    e                                    )                                            ⁢                              (                                                      K                    w                                    /                                      E                    ρ                                                  )                                              ]                    
where c is the propagation velocity of leak signals in the pipe, c0 is the propagation velocity of sound in an infinite body of water equal to {square root over (Kw/xcfx81)}, where Kw is the bulk modulus of elasticity of water, xcfx81 is density of water, Excfx81 is the modulus of elasticity of the pipe material, D is internal diameter of the pipe, e is the thickness of the pipe wall, and xcex1 is constant that depends on the constraints of longitudinal movement of the pipe (xcex1 equals 1 for pipes which are completely free, which is generally the case for pipes having expansion joints. This is normally the case for water distribution pipes).
The pipe wall thickness is back-calculated by substituting in the above equation the measured value of the propagation velocity and values of all other pipe parameters which are usually known. This pipe wall thickness determined in this manner represents an average value the pipe segment between the two points where vibration signals are measured.
The invention can also be used to determine the type of pipe. Sometimes, it is not known what type of pipes are buried, and by measuring the coupled mode propagation velocity it is possible to distinguish between the different types of pipe material, e.g., metallic vs. plastic.
The method is non-destructive and non-intrusive. All required measurements can be made from the ground surface without excavating the pipe or taking it out of service. The pipe wall thickness is an average value. The length of the pipe segment represented by this value can be arbitrarily chosen. Average thickness values are more appropriate and meaningful than discrete values for evaluating the residual life of pipes needed for the long-term planning of rehabilitation and replacement needs. The residual life depends on the overall condition of the pipe rather than on localized defects or thickness variation.
The invention is applicable to both metal and plastic pipes. In the case of plastic pipes, the vibrational frequencies are generally subsonic, whereas in the case of metal pipe they range up to about 800 Hz.
The proposed method is easy to implement, for example, in conjunction with routine leak detection surveys. Also, it does not require a high level of operator skill.